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Versions: 00 01 02 03 04 05 06 07 08 rfc3459                            
Network Working Group                                           E. Burger
Internet Draft                                   Centigram Communications
Document: draft-ietf-vpim-cc-00.txt                            E. Candell
Obsoletes: draft-burger-vpim-cc-00.txt           Comverse Network Systems
Category: Standards Track                                   July 14, 2000
Expires in six Months


                   Critical Content of Internet Mail


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.  Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   One can access the list of current Internet-Drafts at
   http://www.ietf.org/ietf/1id-abstracts.txt

   One can access the list of Internet-Draft Shadow Directories at
   http://www.ietf.org/shadow.html.



1.   Abstract

   This document describes a mechanism for identifying the body parts
   that the sender deems critical in a multi-part Internet mail
   message.

   By knowing what parts of a message the sender deems critical, a MTA
   or UA can intelligently handle multi-part messages.  Applications of
   critical content include, but are not limited to, the following
   applications.  Critical content can help a smart gateway decide what
   parts to forward.  It can indicate how hard a gateway should try to
   deliver a body part.  It can help an MTA to select body parts to
   silently delete when a system of lesser capability receives a
   message.  In addition, critical content can help indicate the
   notification strategy of the receiving system.






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Table of Contents

1.  ABSTRACT .........................................................1
2.  CONVENTIONS USED IN THIS DOCUMENT ................................2
3.  INTRODUCTION .....................................................2

4.  CONTENT-CRITICALITY ENTITY .......................................5
4.1.  CRITICAL .......................................................6
4.2.  IMPORTANT ......................................................6
4.3.  IGNORE .........................................................6
4.4.  Other Values ...................................................7
4.5.  Criticality Precedence .........................................7
4.6.  Summary ........................................................7
5.  BACKWARD COMPATIBILITY CONSIDERATIONS ............................8

6.  SECURITY CONSIDERATIONS ..........................................9
7.  COLLECTED SYNTAX .................................................9
8.  REFERENCES .......................................................9
9.  ACKNOWLEDGMENTS .................................................10
10. AUTHOR'S ADDRESSES ..............................................10


2.   Conventions used in this document

   This document refers generically to the sender of a message in the
   masculine (he/him/his) and the recipient of the message in the
   feminine (she/her/hers).  This convention is purely for convenience
   and makes no assumption about the gender of a message sender or
   recipient.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [2].

   FORMATTING NOTE: Notes, such at this one, provide additional
   nonessential information that the reader may skip without missing
   anything essential.  The primary purpose of these non-essential
   notes is to convey information about the rationale of this document,
   or to place this document in the proper historical or evolutionary
   context.  Readers whose sole purpose is to construct a conformant
   implementation may skip such information.  However, it may be of use
   to those who wish to understand why we made certain design choices.


3.   Introduction

   This document describes the Critical Content identification for
   multi-part Internet mail.

   The need for a critical content identification mechanism comes about
   because of the internetworking of Internet mail systems with legacy


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   messaging systems that do not fulfil all of the semantics of
   Internet mail.  Such legacy systems have a limited ability to render
   all parts of a given message.  This document will use the case of an
   Internet mail system exchanging electronic messages with a legacy
   voice messaging system for illustrative purposes.

   Electronic mail has historically been text-centric.  Extensions such
   as MIME [3] enable the desktop to send and receive multi-part,
   multimedia messages.  Popular multimedia data types include binary
   word processing documents, binary business presentation graphics,
   voice, and video.

   Voice mail has historically been audio-centric.  Many voice
   messaging systems only render voice.  Extensions such as fax enable
   the voice mail system to send and receive fax images as well as
   create multi-part voice and fax messages.  A few voice mail systems
   can render text using text-to-speech or text-to-fax technology.
   Although theoretically possible, none can today render video.

   An important aspect of the interchange between voice messaging
   services and desktop e-mail client applications is that the
   rendering capability of the voice messaging platform is often much
   less than the rendering capability of a desktop e-mail client.  In
   the e-mail case, the sender has the expectation that the recipient
   receives all components of a multimedia message.  This is so even if
   the recipient cannot render all body parts.  In most cases, the
   recipient can either find the appropriate rendering tool or tell the
   sender that she cannot read the particular attachment.

   This is an important issue.  By definition, a MIME-enabled user
   agent, conforming to [4] will present or make available all of the
   body parts to the recipient.  However, a voice mail system may not
   be capable of storing non-voice objects.  Moreover, the voice mail
   system may not be capable of notifying the recipient that there were
   undeliverable message parts.

   The inability of the receiving system to render a body part is
   usually a permanent failure.  Retransmission of the message will not
   improve the likelihood of a future successful delivery.  Contrast
   this to the case with normal data delivery.  Traditional message
   failures, such as a garbled message or disabled link will benefit
   from retransmission.

   This situation is fundamentally different from normal Internet mail.
   In the Internet mail case, either the system delivered the message,
   or it didn't.  There is no concept of a system partially delivering
   a message.

   In addition, the sender would not mind if the system did not deliver
   non-critical parts of a message.  In fact, the sender's user agent
   may be silently adding body parts to a message unbeknownst to the
   sender.  For example, take Microsoft Outlook as a user agent.

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   Outlook often will attach a TNEF section or other body parts. If the
   receiving system rejected the message because it could not render
   TNEF, the sender would be understandably confused and upset.

   Thus, there is a need for a method of indicating to a Mail Transfer
   Agent (MTA) or User Agent (UA) that the sender considers parts of a
   message to be critical.  From the sender's perspective, he would not
   consider the message delivered if the system did not deliver the
   critical parts.

   One method of indicating critical content of a message is to define
   a profile.  The profile defines discard rules based on knowledge of
   the user population for silently deleting body parts.  Citing the
   example above, a voice profile can easily declare that MTAs or UAs
   can silently delete TNEF data and yet consider the message
   successfully delivered.  This is, in fact, the approach originally
   proposed for VPIMv3 [5].

   Since one aspect of the issue is deciding when to notify the sender
   that the system cannot deliver part of a message, one could use a
   partial non-delivery notification mechanism [6] to indicate a
   problem with delivering a given body part.  However, this requires
   the user request a MDN.  Moreover, the sender will receive PNDN
   failures for objects the sender may not be aware he is sending.  An
   example would be the TNEF part.

   Summarizing the needs, we need a mechanism that will let the sender
   or sender's UA mark body parts he considers critical to the message
   that the system must deliver.  The mechanism MUST NOT burden the
   sender with failure notifications for non-critical body parts.  The
   mechanism MUST conform to the general notification status request
   mechanism for positive or negative notification.  When requested,
   the mechanism MUST indicate to the sender when a receiving system
   cannot deliver a critical body part.

   In short, we need a method of letting the sender indicate what body-
   parts he considers to be critical.

   This document describes a Critical Content marking mechanism that
   satisfies these needs.  Following the format for Internet message
   bodies [3], this document introduces the Content-Criticality body
   part header.  Values for this header are CRITICAL, NOTIFY, or
   IGNORE.  The receiving MTA or UA will generate a DSN or PNDN at
   delivery time if it receives a request for notification and the
   (non-)delivery status of the parts marked NOTIFY meet the criteria
   for notification.  Likewise, the receiving UA will generate a MDN or
   PMDN at reading time if it receives a request for notification and
   the (non-)delivery status of the parts marked CRITICAL meet the
   criteria for notification.




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   NOTE: A straight-forward alternative implementation method for
   marking a body part critical is to use a content-disposition
   parameter called "criticality".

   <<<EDITOR'S NOTE: Let's discuss the merits of content-disposition on
   the list.  It's trivial to rework this ID; only the Entity changes.
   That is, rather than having a 'Content-Criticality: Foo' part header
   (entity), we have a 'content-disposition: ... ; criticality="Foo"'
   parameter.  Everything else in the document stands (the parameter
   names and actions).

   I at first proposed content-disposition; Ned feels strongly about
   it; Keith likes it.  Emily felt strongly that the indicator
   shouldn't be content-disposition, but rather its own entity.  I
   can't remember all the reasons, but they were enough for me to go
   down the entity, rather than parameter path.

   Comments?>>>

   <<<EDITOR'S NOTE: We don't have an ID for PMDN yet, but it will look
   like PNDN.  Stay tuned.>>>



4.   Content-Criticality Entity

   The Content-Criticality field is a MIME body part header inserted by
   the sending UA to indicate to the receiving MTA or UA whether to
   consider this body part critical.

   A CRITICAL body part is one the sender requires the receiving system
   to deliver for him to consider the message delivered.

   An IMPORTANT body part is one the sender doesn't consider critical
   to the delivery of the message.  However, the sender may be
   interested in knowing if the receiving system delivered the body
   part.

   An IGNORE body part is one the sender doesn't care about whether the
   receiving system delivers it or not.  The receiving system can
   silently delete such body parts if the receiving system cannot
   deliver the part.

   The terms "entity" and "body part" have the meanings defined in [3].

   One obvious application of critical content is generating a
   (non-)delivery message.  If the value of the field is IGNORE, the
   receiving MTA or UA MUST NOT generate a notification.  If the value
   of the field is CRTITICAL, the receiving MTA or UA will generate a
   notification, based on the normal notification request mechanisms.
   Normal notification request mechanisms include the SMTP RCPT NOTIFY
   command [7] and the Disposition-Notification-To header [8].

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   For example, if the sending system requests notification, and a
   CRITICAL part fails, the receiving system will generate a NDN for
   the whole message.  Conversely, if only an IGNORE part fails, the
   receiving system will not generate a NDN.

   The next sections examine the actions taken by an MTA or UA given
   the different values of Content-Criticality.

   NOTE: This implies that the MTA must examine the entire message on
   receipt to determine whether it needs to generate a notification.
   However, the MTA need not examine the message if it knows it can
   store and forward all media types.  Said differently, an Internet e-
   mail MTA can, by default, handle any arbitrary MIME-encapsulated
   type.  Some voice mail systems, on the other hand, cannot store
   binary attachments at all, such as application/ms-word.  The voice
   mail MTA, in this example, would be scanning for non-renderable body
   parts in any event.

  4.1. CRITICAL

   "Content-Criticality: CRITICAL" signifies that this body part is
   critical to the sender.

   If the receiving system cannot render or store a body part marked
   CRITICAL, then the entire message has failed.  In this case, the
   receiving system MUST take the appropriate failure action.

   NOTE: We say "appropriate action", because the sender may have
   suppressed all notifications.  In this case, the appropriate action
   is to simply discard the message.

  4.2. IMPORTANT

   "Content-Criticality: IMPORTANT" signifies that the sender wishes to
   receive notification reports for this body part.  Failure of such a
   body part does not result in the failure of the message as a whole,
   as happens with CRITICAL body parts.  However, the receiving system
   MUST perform the appropriate body-part failure action.

   If the receiving system cannot render or store a body part marked
   IMPORTANT, the receiving system MUST take the appropriate failure
   action.  It is important to note that if the system is successful in
   delivering other critical body parts, then the message delivery is
   successful.  In this situation, the receiving system MUST return a
   partial non-delivery notification [6].

  4.3. IGNORE

   "Content-Criticality: IGNORE" signifies that the sender does not
   care about notification reports for this body part.


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   If the receiving system cannot render or store a body part marked
   IGNORE, the receiving system may silently delete the body part.  The
   receiving system MUST NOT return a delivery failure, unless parts
   marked IMPORTANT or CRITICAL have also failed.

  4.4. Other Values

   The receiving system MUST treat unrecognized values as IMPORTANT.
   This is to provide backward compatibility with future uses of the
   Content-Criticality entity.

   <<<ALTERNATIVES:

   IGNORE: If I don't recognize something, ignore it!

   CRITICAL: Future values are more likely to involve some sort of
   notification, rather than non-notification.  However, if the
   notifications are more sophisticated than CRITICAL, senders may be
   miffed they didn't get the processing they expected.

   Reject the Message: This would be too extreme!  Yes, it would weed
   out non-conformant sending UAs, but this would really piss off
   users.


   So far, we have one vote for IGNORE, as that would be compatible
   with VPIMv2.
   We have one vote for IMPORTANT, as that would provide forward-
   compatibility with future uses of Content-Criticality.
   We expect this to be a point of discussion on the list.

   End of ALTERNATIVES>>>

  4.5. Criticality Precedence

   <<<We'll fill-in this in the next draft.>>>

  4.6. Summary

   The following table summarizes the actions expected of a conforming
   MTA or UA.












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                            +--------------------------------------+
                            |    Sending UA Has Marked Body Part   |
                            |-------------+-------------+----------|
                            |  CRITICAL   |  IMPORTANT  |  IGNORE  |
       +--------------------+-------------+-------------+----------+
       | Body Part is       | Appropriate | Appropriate |          |
       | Deliverable/read   |   Action    |   Action    |  ignore  |
       +--------------------+-------------+-------------+----------+
       | Body Part is       | Fail Entire | Appropriate |          |
       | Undeliverable /    |   Message   |   Action    |  ignore  |
       |  Unreadable        |             |             |          |
       +--------------------+--------------------------------------+

   The distinction between deliverable/read is as follows.  A MTA can
   determine if a body part is deliverable.  Sample actions a MTA can
   take are generating a NDN or DSN.  An UA can determine if a body
   part is readable.  A sample action an UA can take is generating a
   MDN.

   The "Appropriate Action" is the action the MTA or UA would take
   given the context of execution.  For example, if a sender requests
   return receipt and the receiver reads a CRITICAL body part, the
   receiving UA must generate the appropriate MDN (following the rules
   for MDN).

   "Ignore" means the MTA or UA MUST ignore the operation on the body
   part.  The MTA or UA MUST treat the message as if the body part was
   not present in the message.


5.   Backward Compatibility Considerations

   If there are no Content-Criticality entities in the message, the
   default value for Content-Criticality is CRITICAL.  The standard
   notification mechanisms work for sending user agents (UA) that do
   not know about the content notification entity.  All body parts are
   critical, because they have the default marking of CRITICAL.

   If there is at least one Content-Criticality entity in the message,
   the default value for unspecified body parts is IGNORE.  The
   philosophy is that UAs, especially manually constructed messages,
   will explicitly mark the critical body parts.

   NOTE: We could choose the default value for Content-Criticality to
   be IGNORE.  This would make VPIMv2 automatically compliant with this
   document, as VPIMv2 has provision to silently delete undeliverable
   parts.  However, VPIMv2 systems should not be receiving arbitrary e-
   mail from the Internet.  If they do, they should be compliant with
   this series of documents.  By defaulting to CRITICAL, this draft is
   compliant with the rest of the Internet infrastructure.



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   NOTE: Some VPIMv2 implementations can receive arbitrary e-mail from
   the Internet.  However, these systems are really acting in the
   capacity of an Internet Voice Mail system.  In this case, one would
   expect the implementation to provide Internet Voice Mail semantics
   to Internet Voice Mail messages.


6.   Security Considerations

   Receiving systems and users should not place any authentication
   value on the Content-Criticality entity.  Just because a message has
   a particular Content-Criticality value doesn't mean that the message
   really originated at a given type of system.


7.   Collected Syntax

   The format of the collected syntax is in accordance with the ABNF of
   [9].  Note that per RFC 2045, none of the strings are case
   sensitive.


        "Content-Criticality" ":" notification-type CRLF

        notification-type = "CRITICAL" / "IMPORTANT" / "IGNORE"



8.   References


   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

   3  Freed, N. and Borenstein, N., "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, Innosoft and First Virtual, November 1996.

   4  Freed, N. and Borenstein, N., "Multipurpose Internet Mail
      Extensions (MIME) Part Two: Media Types", RFC 2046, Innosoft and
      First Virtual, November 1996.

   5  Vaudreuil, G. and Parsons, G., "Voice Profile for Internet Mail -
      version 3", <draft-ema-vpimv3-00.txt>, Work in Progress, expired.

   6  Burger, E., "Partial Non-Delivery Notification", Work in
      Progress, draft-ema-burger-pndn-01.txt, March 2000.



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   7  Moore, K., "SMTP Service Extension for Delivery Status
      Notifications", RFC 1981, University of Tennessee, January 1996.

   8  Fajman, R., "An Extensible Message Format for Message Disposition
      Notifications", RFC 2298, National Institutes of Health, March
      1998.

   9  Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, Internet Mail Consortium and
      Demon Internet Ltd., November 1997.




9.   Acknowledgments

   We'd like to thank Ned Freed for pointing out that this mechanism
   was about criticality, not notification per se.  That insight made
   the concept and descriptions infinitely more straight forward.  If
   it's still confusing, it's our fault!

   We'd also like to thank Keith Moore for helping us tighten-up our
   explanations.


10.    Author's Addresses

   Eric Burger
   Centigram Communications Corporation
   Maryland Technology Center
   1375 Piccard Dr., MS 150I
   Rockville, MD  20850-4311
   USA

   Phone: +1 301/212-3320
   Email: e.burger@ieee.org


   Emily Candell
   Comverse Network Systems
   200 Quannapowitt Pkwy.
   Wakefield, MA  01880
   USA

   Phone: +1 781/213-2324
   Email: emily@comversens.com






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